Simferopol, Ukraine: Ukraine put its military on high alert on Sunday in response to Russia's move to seize control of the Crimean peninsula, and it threatened war against Moscow if the Kremlin made further incursions into Ukrainian territory as Western powers scrambled to find a response to the crisis.

"This is the red alert, this is not a threat, this is actually a declaration of war to my country," Ukraine's Prime Minister, Arseniy Yatsenyuk, said after Russia's parliament approved the deployment of troops to any part of Ukraine where Moscow deems Russians are in danger. He warned that Ukraine was on the "brink of disaster" and appealed to the international community to stand by Kiev.

Armed men stand guard at the local government headquarters in Simferopol, Crimea. Photo: Reuters

Ukraine launched a treason case on Sunday against the head of its navy, who surrendered his headquarters in the Crimean port of Sevastopol on only his second day in the job.

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As Ukrainians gathered in central Kiev for a large patriotic rally, Russian troops without badges and pro-Russian "self-defence" forces surrounded some Ukrainian military bases in Crimea, urging the soldiers and sailors to defect.

Ukrainian navy chief Denis Berezovsky was shown on Russian television swearing allegiance to the new pro-Russian regional leaders of Crimea, who are not recognised by Kiev. Russian forces have seized the Black Sea peninsula and told Ukrainian forces there to give up their weapons.

Pro-Russian activists vote during a rally in the centre of the eastern Ukrainian city of Donetsk. Photo: AFP

Although all of Ukraine's armed forces are on highest alert, Kiev's small and underequipped military is seen as no match for Russia's superpower might. The Defence Ministry was also ordered to stage a call-up of reserves - theoretically all men aged up to 40 in a country with universal male conscription, although Ukraine would struggle to find extra guns or uniforms for significant numbers of them.

The new government in Kiev, hastily installed last week, wants to influence the thinking of Russian President Vladimir Putin, an effort expected to continue when NATO holds an emergency meeting on Ukraine and British Foreign Secretary William Hague visits Kiev in a sign of Western support.

US Secretary of State John Kerry on Sunday called Russia's behaviour "an act of aggression that is completely trumped up", suggesting that Russia was "possibly trying to annex Crimea". But while saying Russia was "in direct, overt violation of international law", Mr Kerry tempered his vigorous denunciation by saying that "the last thing anybody wants is a military option in this kind of a situation".

People gather for a rally against Russian intervention in Ukraine in central Kiev's Independence Square. Photo: AFP

Despite a 90-minute phone call between US President Barack Obama and Mr Putin at the weekend, and other calls to the Kremlin by European leaders, Russia shows no sign of backing down.

German Chancellor Angela Merkel accused Mr Putin of breaching international law with "unacceptable Russian intervention" in Ukraine, a German government spokesman said on Sunday after a phone call between the two leaders.

However, Mr Putin accepted Dr Merkel's proposal to establish a "fact-finding mission" such as a contact group, possibly under the leadership of the Organisation for Security and Co-operation in Europe, to start a political dialogue.

Mr Kerry said Russia could pay "a huge price" for its behaviour, including the possible cancellation of the Group of Eight meeting slated for June in Sochi. If this continues, he said, Mr Putin "is not going to have a Sochi G8, he may not even remain in the G8". Britain has suspended its participation in preparations for the G8 meeting, the US has already said it will not take part in the meetings, and a source in French President Francois Hollande's office said Paris has also pulled out.

Before the emergency NATO meeting, its secretary-general, Anders Fogh Rasmussen, told Russia to stop its military activity and threats against a sovereign Ukraine. "What Russia is doing now in Ukraine violates the principles of the United Nations Charter," he said. "It threatens peace and security in Europe. Russia must stop its military activities and its threats."

What began three months ago as a protest against the Ukrainian government has now turned into a big-power confrontation reminiscent of the Cold War and a significant challenge to international agreements on the sanctity of the borders of the post-Soviet nations.

In Crimea, the situation was calm but hardly placid on Sunday morning, with fewer soldiers visible on the streets. Some heavily armed soldiers without insignia had taken up positions around small Ukrainian military bases, but did not try to enter them.

At Perevalnoye, a small Ukrainian base some 24 kilometres south of Simferopol on the road to Yalta, hundreds of soldiers with masks, helmets and goggles, in unmarked uniforms, surrounded the base using vehicles with Russian plates. Inside about two dozen Ukrainian soldiers could be seen, equipped with an old BMP, a combination of a light tank and armoured personnel carrier.

The Ukrainian commander, Colonel Sergei Starozhenko, told reporters the unmarked troops had arrived about 5am and "they want to block the base". He said he expected them to bring reinforcements and call for talks. Asked how many men he has at his command, he said, "Enough".

After 15 minutes of conversation with what appeared to be a Russian officer, he said, "there won't be war", and returned inside, while the standoff continued.

On Saturday, Russia took effective control of Crimea. Russian troops without identifying insignia but using military vehicles bearing the licence plates of Russia's Black Sea force encircled government buildings, closed the main airport and seized communication hubs, solidifying what began on Friday as a covert effort to control the largely pro-Russian region.

According to Kiev, Russia then flew in 6000 more troops to a military airport. The civilian airports are now open again.

In Moscow, Mr Putin on Saturday convened the upper house of parliament to grant him authority to use force to protect Russian citizens and soldiers not only in Crimea but throughout Ukraine. Both actions - military and parliamentary - were a direct rebuff to Mr Obama, who on Friday pointedly warned Russia to respect Ukraine's territorial integrity.

Mr Obama accused Russia on Saturday of a "breach of international law" and condemned the country's military intervention, calling it a "clear violation" of Ukrainian sovereignty.

Russia kept up its propaganda campaign on Sunday in defence of the takeover, citing undefined threats to Russian citizens and proclaiming "massive defections" of Ukrainian forces in Crimea, which Western reporters said appeared to be unfounded.

All eyes are now on whether Russia makes a military move in predominantly Russian-speaking eastern Ukraine, where pro-Moscow demonstrators have marched and raised Russian flags over public buildings in several cities including Kharkiv, Donetsk, Odessa and Dnipropetrovsk in the last two days.

Russia has staged war games with 150,000 troops along the land border, but so far they have not crossed. Kiev says Moscow is orchestrating the protests to justify a wider invasion.

At the United Nations, the Security Council on Saturday held an emergency meeting on Ukraine for the second time in two days.

UN Secretary-General Ban Ki-moon also spoke with Mr Putin on Saturday and described himself as "gravely concerned" and urged Mr Putin to negotiate with officials in Kiev.